The Growth of Cloud Drops by Condensation. II. The Formation of Large Cloud Drops

Abstract

The observed stability of many deep non-freezing clouds suggests that there is a gap between the sizes normally reached by droplets due to condensation and the sizes at which coalescence of drops becomes effective in producing rain. One way such clouds could yield rain would be for condensation to give rise to some drops bigger than usual. Conditions for the growth of some drops to a relatively large size by condensation are investigated theoretically. This requires that in some air parcels an undue proportion of the available water vapour should condense on relatively few nuclei. The types of vertical motion and nucleus spectra which could yield this result are investigated. Slow motions are favourable. Suitable spectra are : (i) a few " giant ' nuclei (M = 10-10, 10-12) ; (ii) all but a few nuclei relatively small. Rain itself tends to produce type (ii) spectrum, so that it could perhaps act regeneratively. It is found that very large cloud drops can be formed by condensation under conditions which, on present knowledge, seem likely to occur at times, but not normally. The hypothesis that the factors studied here largely determine colloidal stability in non-freezing cloud is therefore satisfactory, inasmuch as it leaves room for both stability and instability.